1. Field of the Invention
The present invention relates to disk drives. In particular, the present invention relates to a disk drive measuring pole tip protrusion (PTP) by performing a write operation to heat the transducer.
2. Description of the Prior Art
FIGS. 1A and 1B show a prior art head disk assembly (HDA) comprising a disk 2 having a 11 disk surface 4. The disk 2 is rotated about a center axis while a transducer 6 writes and reads data to/from the disk surface 4. The transducer 6 is mounted in a slider which is attached to the distal end of an actuator arm 8. The actuator arm 8 is rotated about a pivot by a voice coil motor (VCM) 10 in order to actuate the transducer 6 radially over the disk surface 4. The disk surface 4 comprises a number of radially spaced, concentric tracks 12 as defined by a number of embedded servo sectors 14. The embedded servo sectors 14 are read by the transducer 6 and processed by a servo control system in order to maintain the transducer 6 over a center of a target track while writing or reading data. The disk drive may employ top and bottom transducers for reading/writing both the top and bottom surfaces of the disk 2.
As the disk 2 rotates, an air bearing forms between the disk surface 4 and the transducer 6 such that the transducer 6 “flies” just above the disk surface 4. It is desirable to minimize the fly height to achieve high recording densities since the read signal strength increases as the gap between the transducer 6 and disk surface 4 decreases. However, if the transducer 6 actually contacts the disk surface 4 during operation it can result in medium defects as well as undesirable head wear. The physical characteristics of the slider, disk 2, and actuator arm 8, are therefore designed to achieve a desired fly height while minimizing the occurrence of head/disk contact. To facilitate the design process, various in-situ techniques have been employed to measure the fly height of production disk drives. The fly height measurement may also be used to identify and discard or repair marginal disk drives during the manufacturing process as well as to predict drive failure while in the field.
U.S. Pat. No. 4,777,544 discloses a harmonic ratio fly height measurement technique wherein a data sequence is written to the disk which upon readback has a spectrum that is constant along the track and which has non-zero amplitude for at least two different frequencies. For example, a data sequence is written to the disk such that the readback spectrum comprises a fundamental frequency and odd harmonics. The fly height is then measured by computing the natural logarithm of the ratio between the fundamental frequency and the third harmonic frequency at two different fly heights (two different angular velocities). Although this technique provides an adequate fly height measurement of a static head/disk interface, it does not account for changes in fly height due to pole tip protrusion. Pole tip protrusion occurs when the part of the slider expands due to heating during a write operation, wherein the resulting protrusion can cause the slider to contact the disk surface.
There is, therefore, a need to measure pole tip protrusion in a disk drive to facilitate design modifications, manufacture screening, and drive failure prediction.